This invention relates to fluid compositions whose viscosity can be carefully modulated from very low viscosity to sufficient viscosity to act as a barrier to further flow. More specifically, the invention relates to diverting fluids used for stimulating hydrocarbon-bearing formations i.e., to increase the production of oil/gas from the formation.
Hydrocarbons (oil, natural gas, etc.) are obtained from a subterranean geologic formation (i.e., a “reservoir”) by drilling a well that penetrates the hydrocarbon-bearing formation and thus causing a pressure gradient that forces the fluid to flow from the reservoir to the well. Often, a well production is limited by poor permeability either due to naturally tight formations or to formation damages typically arising from prior well treatment, such as drilling, cleaning etc.
To increase the net permeability of a reservoir, it is common to perform a well stimulation. The most common stimulation technique consists of injecting an acid that reacts with and dissolves the damage or portion of the formation thereby creating alternative flowpaths for the hydrocarbons to migrate through the formation to the well. This technique known as acidizing may eventually be associated with fracturing the injection rate and pressure is enough to induce the formation of a fracture in the reservoir.
Fluid placement is critical to the success of stimulation treatment. Natural reservoirs are often heterogeneous; the acid fluid will preferentially enter areas of higher permeability in lieu of treating the areas where a treatment is most needed. Similarly, acid treatment tends to remove the damages that are easier to reach, due to a lower degree of damage or higher permeability. Each additional volume of acid fluid follows the path of less resistance, failing to remove the most important damages. The critical character of fluid placement is exacerbated by the fact that the acid reacts extremely quickly and consequently, is spent almost immediately and therefore not available for treating the untreated areas.
In order to control placement of treating fluids, various techniques have been employed. Mechanical techniques involve for instance the use of ball sealers and packers and of coiled tubing placement to specifically spot the fluid across the zone of interest. Non mechanical techniques typically make use of gelling agents as diverters for temporary impairing the areas of higher permeability and increasing the proportion of the treating zone that goes into the areas of lower permeability. Of course, a diverter should not itself damage the reservoir and therefore it is important that it can be easily removed following the acid treatment so that the zones of higher permeability remain so.
Most commercialized chemical diverters are based on cross-linked polymers. Unfortunately, these systems leave residues in the formation, which can damage the formation, resulting in diminished hydrocarbon production. In addition, the cross-linking reaction is easily perturbed by formation chemistry, contaminants in the pumping equipment, and so forth.
It is also known to use self-diverting acids, typically consisting of hydrochloric acid mixed with a gelling agent and a pH-sensitive cross-linker. Self-diverting acids are typically designed to gel at intermediate pH values, when the acid is partially spent. Self-diverting systems not based on cross-linking chemistry by which rely upon viscoelastic surfactants are described in U.S. Pat. No. 4,695,389 (see also, U.S. Pat. No. 4,324,669, and British Patent No. 2,012,830, both cited there) which has a common assignee as the present application. Viscoelastic surfactants based systems exhibit very low friction pressure and therefore are easy to pump and yet, form a gel downhole. U.S. Pat. No. 4,695,389 discloses a viscoelastic surfactant-based gelling agent intended for use in acid fracturing. The particularly preferred embodiment is a fluid comprised of N,N-bis(2-hydroxyethyl) fatty amine acetic acid salt (the gelling agent), an alkali metal acetate salt, acetic acid (the acid which actually removes the damage from the formation), and water.
Improved self-diverting systems have been described in U.S. patent application Ser. No. 09/419,842, having a common assignee as the present application and its corresponding International Patent Application WO 01/29369. This application, hereby incorporated by reference, provides formulations suitable for acid treatment comprising an amphoteric surfactant that gels as the acid spends in presence of an activating amount of a co-surfactant and of multivalent cations typically generated by the acid reaction with the formation. When the gelling agent is mixed in hydrochloric acid, the co-surfactant prevents the gelling of the solution; the solution gels when the pH increases above 2.
In a preferred embodiment, the amphoteric surfactant is oleylpropyl betaine of the formula:
and the co-surfactant is preferably sodium dodecyl benzene sulfonate.
Compositions known from the U.S. patent application Ser. No. 09/419,842, and its corresponding International Patent Application WO 01/29369, have been of limited commercial applications due to temperature limitations. In particular, formulations based on oleylamidopropyl betaine and sodium dodecyl benzene sulfonate can only be used for temperature below about 200° F. (93° C.) while the carbonate acidizing market calls for temperature up to about 300° F. (about 150° C.).
According to a first embodiment of the present invention, there is provided a method for stimulating a subterranean hydrocarbons reservoir comprising contacting the formation with a treating fluid consisting of an acid, erucylamidopropyl betaine (or a protonated, deprotonated, homolog or salt thereof), and an-alcohol, typically selected among methanol and/or ethanol, and most preferably being methanol. The formulation of the present invention may also comprise various standard additives such as corrosion inhibitors, non-emulsifiers and iron control agents. One embodiment is a method of treating a subterranean formation involving contacting the formation with a treating fluid containing an aqueous solution, an acid selected from an organic acid at a concentration of greater than 12% and an inorganic acid, and a surfactant acting as gelling agent essentially consisting of erucylamidopropyl betaine, or a protonated/deprotonated homolog or salt thereof, without a co-surfactant; preferably, the acid is present in the fluid at a concentration of at least 15% by weight.